1. Field of the Invention
This invention relates to preparation of improved zeolite-based catalysts especially useful for promoting the alkylation of mono-alkyl aromatic compounds to form a dialkyl substituted aromatic product enriched in the para (i.e., 1,4-)dialkyl benzene isomer. The invention also relates to a process for the alkylation, e.g., ethylation, of monoalkyl aromatic compounds, e.g. toluene or ethylbenzene, in a manner so as to maximize the production of the product para-isomer, to minimize the production of the product meta isomer and to substantially eliminate the production of the product ortho isomer.
2. Description of the Prior Art
Zeolite-containing compositions are well known catalysts for promoting conversion of aromatic hydrocarbons to dialkyl substituted aromatic compounds via alkylation, transalkylation, disproportionation and isomerization reactions. Numerous techniques are also known for modifying zeolite-based aromatics conversion catalysts of this type in order to provide catalysts which promote production of a reaction product which is enriched in the para-isomer of the desired disubstituted aromatic material. For example, Kaeding, U.S. Pat. No. 4,117,024, Issued Sept. 26, 1978 and assigned to Mobil Oil Corporation, the assignee of the instant invention, discloses a process for the conversion of toluene and/or ethyl benzene to its corresponding para ethyl alkylation product by carrying out the alkylation in the presence of hydrogen and using as a catalyst a crystalline aluminosilicate zeolite of specified acidity, sorption characteristics and Constraint Index. U.S. Pat. No. 4,117,024 discloses many materials which exemplify this genus of catalysts including, but not limited to, ZSM-5, ZSM-11, ZSM-12, ZSM-35, and ZSM-38. The patent also discloses that the zeolite material may be modified in one or more ways to improve the para-selectivity properties of the catalyst. This U.S. Pat. No. 4,117,024 is incorporated herein in its entirety by reference as setting forth applicable prior art relative to this invention.
Additionally, reference is further made to Kaeding and Young, U.S. Pat. No. 4,034,053, Issued July 5, 1977; Kaeding, U.S. Pat. No. 4,049,573, Issued Sept. 20, 1977; and Kaeding and Young, U.S. Pat. No. 4,086,287, Issued Apr. 25, 1978. All of these patents are also incorporated herein by reference in their entirety as setting forth additional applicable prior art involving the modification of zeolite based catalysts of this same general type in order to improve the para-selectivity characteristics thereof when such materials are used to promote various aromatic hydrocarbon conversion reactions, including alkylation of monoalkyl substituted aromatics to produce dialkylbenzene compounds.
Considering all of such prior art references together, a process has been designed for the commercial production of para-ethyltoluene by the catalytic ethylation of toluene with ethylene using cofed hydrogen. Such a process utilizes what was heretofore believed to be the best catalyst for maximizing para-isomer, minimizing meta-isomer, eliminating ortho isomer, providing high conversion of reactants to products and permitting low catalyst aging rate. This optimized prior art catalyst is a crystalline siliceous material of ZSM-5 topology, as characterized by significant x-ray diffraction pattern lines, which is composited with a binder and is then impregnated with both phosphorus and magnesium. This selected catalyst is made by a series of process steps comprising: preparing the siliceous crystalline zeolite; binding the zeolite with a matrix material, suitably alumina; steaming the resulting zeolite-containing composite; impregnating the composite with diammonium phosphate followed by filtering, drying and calcining the resulting phosphorus-impregnated composite; contacting the P-containing composite in a first magnesium impregnation stage with a magnesium acetate solution, followed by calcination; thereafter contacting the composite, in a second separate magnesium impregnation stage, with another batch of magnesium acetate solution, followed again by calcination to prepare the final from catalyst. The modified zeolite catalyst produced in this manner is well suited to use in the toluene ethylation process. As can be seen from the data presented in the referenced U.S. Pat. No. 4,117,024, an ethyl toluene product is thus produced having desirable isomeric distribution characteristics, with very advantageous catalyst life and conversion capability.
From the foregoing preparation description and referenced data, it can be seen that the prior art catalyst selected as the best for commercialization, i.e. a magnesium and phosphorus impregnated, alumina-bound zeolite material, achieves its best selectivity for ethylene alkylation of toluene to para-ethyltoluene at impregnant loadings of 7 and 3 weight percent respectively for magnesium and phosphorus, provided the catalyst composite into which these materials are impregnated is presteamed. Without wishing to be bound by theory, it is believed that the magnesium being impregnated onto such a catalyst can have a significant affinity for the binder portion of the catalyst composite. It is further believed that the initial treatment of the prior art catalyst composite with the phosphorus impregnant serves to "passivate" the binder material, thereby promoting greater association of magnesium with the zeolite portion of the composite upon subsequent treatment of the composite with the magnesium acetate solution. Since it is expected that it is magnesium associated with the zeolite material in such composite which provides the excellent selectivity characteristics of such prior art composites for production of para-ethyltoluene, the phosphorus followed by magnesium treatment of such composites serves to provide highly desirable toluene alkylation catalysts.
Notwithstanding the suitability of such prior art Mg-P-ZSM-5 type zeolite catalyst composites for use in the commercial-scale production of para-ethyltoluene, there are still several disadvantages associated with the large scale preparation of catalysts of this type in the manner described. For example, if the impregnated catalyst is not presteamed, para-ethyltoluene selectivity may not be as high as needed for some commercial production operations. Furthermore, magnesium impregnation concentration to the optimum 7 weight percent cannot generally be achieved during commercial scale catalyst production, when using a magnesium acetate impregnant solution, in a single impregnation. Multiple impregnations, with intermediate calcination, are usually required during commercial scale production to achieve the requisite 7% concentration of magnesium. Still further, even to achieve this result using multiple impregnations, it is necessary to use very concentrated aqueous magnesium acetate solutions, e.g. about 50 to 60 weight percent in water. Such solutions are very viscous and thus have to be utilized as impregnants at elevated temperatures, e.g. about 150.degree. F., in order to reduce impregnant viscosity to acceptable impregnation levels.
All of the foregoing recited disadvantages of the previously selected optimum magnesium/phosphorus based alkylation catalyst composites should not be taken to in any way mean that such a catalyst was or is unsatisfactory. Quite to the contrary, such a prior catalyst is excellent, far superior to it predecessors and is quite well suited to use in the aromatics alkylation processes described. It is furthermore commercially manufacturable, albeit with some difficulty and expense. Notwithstanding the suitability of such prior art alkylation catalysts, there is nevertheless a continuing need to develop additional catalysts, catalyst preparation procedures and alkylation processes employing such catalysts which provide one or more performance or commercial advantages over similar catalysts, procedures and processes of the prior art.
Accordingly, it is an object of the present invention to provide an additional type of zeolite-based catalyst composite suitable for promoting the para-selective conversion of monoalkyl aromatics such as toluene to dialkylbenzene materials such as para-ethyltoluene. It is a further object of the present invention to provide such an additional type of alkylation catalyst which is substantially phosphorus-free but which nevertheless exhibits selectivity and activity characteristics comparable or superior to those of the hereinbefore described magnesium phosphorus alkylation catalysts of the prior art. It is further an object of the present invention to provide such an additional type of alkylation catalyst which, in comparison with preferred prior art catalysts is simpler and easier to manufacture via a novel method for catalyst preparation. It is a further object of the present invention to provide an aromatics alkylation process employing such an improved zeolite-based alkylation catalyst.